The American Petroleum Institute (API) has standardized various thread forms that are used in the drilling and production of oil and gas wells. One API thread form commonly found in connections used to secure drill pipe together is a modified V-type thread that has a root radius of 0.038 in. The included angle between the stab and load flanks of the thread is 60 degrees and the thread root cutting radius of 0.038 in. is centered on the bisector of the included angle. The API connection provides a root truncation of the straight-V thread form of 0.038 in. where the root truncation is a measure of the distance between the apex of the 60 degree included angle and the thread root.
The performance of the API connection has been improved in the prior art by modifying the thread root configuration. One prior art improvement has been to increase the thread root radius from 0 0.038 in. to 0.042 in. Another prior art improvement has been to enlarge the root by forming different surfaces of revolution along the flank and adjoining root surfaces using different cutting radii centered on the stab flank side of the included angle bisector. This latter thread form is more fully described in U.S. Pat. Nos. 4,549,754 and 6,467,818 incorporated herein in their entirety.
The preferred thread forms described in the '754 patent provide for two different size root cutting radii with the larger of the cutting radii to be substantially larger than the root truncation.
A specific illustrated form of the threads employs one cutting radius of 0.057 in. and another of 0.031 in. within a thread form having a usual root truncation of 0.038 in. as required for mating with an API connection. In the preferred configuration, the smaller radius represents approximately 54 percent of the length of the larger radius.
The '754 patent recognizes that any increase of the root radius over root truncation will result in a decrease in maximum stress and that a substantial increase is preferred to obtain a substantial decrease in maximum stress. The '754 patent proposes as a limit to the root radius increase that the root radius should not be increased to a point that is greater than a distance that would intersect or exceed the pitch diameter. The '754 Patent notes, however, that it is difficult to use this limit of enlarged root diameter in practice since it would tend to produce an under-cut area that would make it difficult to use standard cutting tools. The preferred thread form in the '754 patent was also dictated by a desire to reduce the original thread shear area (base width of the thread form) preferably by not more than 25 percent.
The objectives of the '754 patent were achieved by joining the unloaded thread flank (stab flank) to the thread root by a planar surface with the smaller radius curvature bridging the planar surface and the larger radius root surface.
The thread types of the '754 patent are particularly well-suited to be employed as threaded connectors for use in drilling and producing oil and gas wells. Such connectors are assembled by rotating one of the pipe bodies relative to the other causing the threads to mate and engage with each other to hold the two pipe bodies together. When these connectors are used to assemble a drill pipe and other drilling assemblies, the connections are repeatedly made up and broken out as a part of the process of running the pipe into and out of the well. The speed of assembling and disassembling the connections and the amount of thread wear involved in the process are functions of the number of revolutions of the pipe required to effect full connection and release of the threaded components.
Thread make up speed and thread wear can be reduced by reducing the total amount of thread engagement required for a fully made up connection. The mechanical strength of the connection is, however, also reduced when the total thread engagement is reduced. U.S. Pat. No. 1,614,815 discloses a tubular coupling with double start threads which are mutilated (circumferentially segmented). One of the shoulders is also tapered relative to a plane perpendicular to the connection's central axis. U.S. Pat. No. 2,885,225 discloses a double start pipe coupling without torque shoulders. U.S. Pat. No. 4,717,183 discloses a multi-start threaded connection with a small angle load flank for the threads.
More recently, U.S. Pat. No. 6,120,067 discloses a large diameter threaded tool joint with dual mating shoulders. Publication US 2004/0100098 also discloses a thread design with multi-start threads.